The present invention relates to a fabrication method of thin film transistor liquid crystal display (TFT-LCD) and, more particularly, to a crystallization method of polysilicon (poly-Si) film in thin film transistors.
General active-matrix LCDs can be divided into two types according to adopted material: one is poly-Si TFT type, and the other is amorphous silicon TFT type, wherein the poly-Si TFT type can provide higher reliability and reduce the cost because it can be integrated with driving circuits. Nonetheless, the main reason why the poly-Si TFT technology is highly commended is that the device size can be greatly reduced to achieve high resolution. In general, to massively product poly-Si TFT-LCDs with low-temperature (about 450xcx9c550xc2x0 C.) fabrication technique, there are several essentials: low-temperature formation of high-quality poly-Si thin film and gate-insulator, large-area ion doping and activation technique, and hydrogenation technique.
For the formation of poly-Si thin film in a TFT-LCD, low temperature technique is adopted for thin film growth m consideration of price of glass substrate. Therefore, solid phase crystallization (SPC) is first introduced. However, its process temperature is still too high. The process temperature is about 600xc2x0 C. and the crystallization quality is not good. Hence, excimer lasers are applied to the above low-temperature process of thin film crystallization. Generally speaking, because there is no specific crystalization direction for amorphous Si, very flat surface of amorphous silicon thin film can be deposited. Therefore, a layer of amorphous silicon thin film is first deposited and then crystallized into a poly-Si thin film using SPC or excimer lasers for fabricating poly-Si thin film transistors.
FIGS. 1 and 2 show a prior art fabrication flowchart of poly-Si thin film using excimer laser crystallization in a poly-Si TFT.
Firsty, please refer to FIG. 1, a substrate 100 having an insulator layer 102 thereon is provided. A flat amorphous silicon layer 104 is then formed on the insulator layer 102 by means of low-pressure chemical vapor deposition (LPCVD), plasma-enhanced chemical vapor deposition (PECVD), or sputtering.
Please refer to FIG. 2, after the amorphous silicon layer 104 is deposited, an excimer laser 108 of sufficient energy is used to crystallize amorphous silicon layer 104 to polycrystalline silicon. For acquiring large grain, amorphous silicon layer 104 is almost completely melted. Some non-melted amorphous silicon particles will remain at the interface between the amorphous silicon layer 104 and the insulator layer 102. Next, the melted amorphous silicon layer 104 will crystallize into a poly-Si layer 106 using the non-melted amorphous silicon particles as crystallization seeds. This poly-Si layer 106 is used as the source/drain region and channel region of the TFT.
In prior art, during the low-temperature crystallization procedure of amorphous silicon layer using an excimer laser, the energy density of the excimer laser must be carefully considered to let the amorphous silicon layer be almost completely melted while keeping the flatness of its surface. When the amorphous silicon layer is almost completely melted, some non-melted amorphous silicon particles will remain at the interface between the amorphous silicon layer and the insulator layer. The melted amorphous silicon layer will crystallize into a poly-Si layer using the non-melted amorphous silicon particles as discrete seeds. However, because the excimer layer is a kind of pulse laser, there will exist slight difference in energy density for each laser pulse. Because it is difficult to control the energy density of the excimer laser, if the energy density of the excimer laser is slightly larger than the ideal value, total melting of the amorphous silicon layer will easily arise, resulting in disappearance of discrete seeds which the crystallization process relies on. Therefore, grain growth will be homogeneous so that the grown grain is small and the homogeneity is bad, as shown in FIG. 13.
Contrarily, if the energy density of the excimer laser is slightly smaller than the ideal value, part of the amorphous silicon layer will not be melted, and the crystallizing grain will grow vertically using the non-melted amorphous silicon layer as the basis, resulting in the same drawbacks of small grain and bad homogeneity.
In prior art, during the crystallization procedure of poly-Si layer, the energy density of the excimer laser must be exactly controlled to let the amorphous silicon layer be almost completely melted and to keep some non-melted amorphous silicon particles remained as discrete seeds for crystallization. Thereby, better effect of crystallization can be obtained. However, because the excimer layer is a kind of pulse laser, there will exist slight difference in energy density for each laser pulse. Because it is difficult to control the energy density of the excimer laser, the process window will become very small.
The present invention aims to propose a method of using an excimer laser to let an amorphous silicon layer having different thickness crystallize into a poly-Si layer so that the above drawbacks of prior art can be overcome.
The present invention proposes a crystallization method of poly-Si thin film in the TFT. The proposed method is briefly described below.
A substrate having an insulator layer is provided. A layer of amorphous silicon having two thickness is first formed on the insulator layer. The region of thinner is defined as the channel region of the TFT, while the region of thicker can be defined as the source/drain regions of the TFT. Next, an excimer laser is used for crystallization. During the excimer laser irradiation, the amorphous silicon layer of thinner is completely melted, and the amorphous silicon layer of thicker is partially melted. The partially melted amorphous silicon layer is used as crystallization seeds. Through formation of the temperature gradient between the completely melted amorphous silicon layer and the partially melted amorphous silicon layer, longitudinal growth of silicon grains in the completely melted region will be performed to grow a poly-Si layer having good homogeneity and large grains.
The various objects and advantages of the present invention will be more readily understood from the following detailed description when read in conjunction with the appended drawings, in which: